Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

The refrigerant enters the compressor as a vapor and is compressed to the

condenser pressure. It leaves the compressor at a relatively high temperature

and cools down and condenses as it flows through the coils of the condenser

by rejecting heat to the surrounding medium. It then enters a capillary tube

where its pressure and temperature drop drastically due to the throttling effect.

The low-temperature refrigerant then enters the evaporator, where it evapo-

rates by absorbing heat from the refrigerated space. The cycle is completed as

the refrigerant leaves the evaporator and reenters the compressor.

In a household refrigerator, the freezer compartment where heat is absorbed

by the refrigerant serves as the evaporator, and the coils usually behind the

refrigerator where heat is dissipated to the kitchen air serve as the condenser.

A refrigerator is shown schematically in Fig. 6–20. Here QLis the magni-

tude of the heat removed from the refrigerated space at temperature TL,QH

is the magnitude of the heat rejected to the warm environment at tempera-

ture TH, and Wnet,inis the net work input to the refrigerator. As discussed

before,QLand QHrepresent magnitudes and thus are positive quantities.

Coefficient of Performance

The efficiencyof a refrigerator is expressed in terms of the coefficient of

performance(COP), denoted by COPR. The objective of a refrigerator is to

remove heat (QL) from the refrigerated space. To accomplish this objective,

it requires a work input of Wnet,in. Then the COP of a refrigerator can be

expressed as

(6–7)

This relation can also be expressed in rate form by replacing QLby Q

.

Land

Wnet,inby W

.

net,in.

The conservation of energy principle for a cyclic device requires that

Wnet,inQHQL¬¬ 1 kJ 2 (6–8)

COPR

Desired output

Required input



QL

Wnet,in

288 | Thermodynamics

CONDENSER

EXPANSION

VALVE

120 kPa

–25°C

120 kPa

–20°C

800 kPa

30 °C

800 kPa

60 °C

COMPRESSOR

QL

QH

Wnet,in

Surrounding medium

such as the kitchen air

Refrigerated space

EVAPORATOR

FIGURE 6–19

Basic components of a refrigeration
system and typical operating
conditions.

Warm environment

at TH > TL

Cold refrigerated

space at TL

R

Wnet,in

QH

QL

Required

input

Desired

output

FIGURE 6–20

The objective of a refrigerator is to
remove QLfrom the cooled space.